1. Field of the Invention
This invention relates to a nitride semiconductor wafer carrying obverse/reverse marks for distinguishing an obverse surface from a reverse surface. Semiconductor nitrides mean gallium nitride (GaN), indium nitride (InN) and aluminum nitride (AlN). The nitrides have wide bandgaps, which give transparency to the nitrides. Gallium nitride (GaN) substrate wafers, for example, are entirely transparent for visible light. Transparency incurs difficulty about making obverse/reverse marks unlike silicon (Si) wafers or gallium arsenide (GaAs) wafers which are opaque for visible light. If a transparent wafer wears a mark on an obverse surface, one can see the mark through a transparent medium from a reverse side. If a mark is scribed on a bottom, the mark can be seen from a top. In the case of a circular wafer, an obverse surface can be distinguished from a reverse surface by making an orientation flat (OF) and identification flat (IF) by cutting two arc fragments with different lengths from a periphery. However, the technology of making GaN crystals is not fully matured. The immaturity prohibits the current technology from producing a large-sized circular GaN wafer. Available best GaN wafers are at present one inch φ (25 mm φ) to two inch φ (50 mm φ) at most.
This application claims the priority of Japanese Patent Application No. 2003-89935 filed on Mar. 28, 2003 and Japanese Patent Application No. 2003-275934 filed on Jul. 17, 2003, which are incorporated herein by reference.
Since round GaN wafers are difficult to produce, rectangle GaN wafers of a side of 1 cm to 2 cm are made. A rectangular wafer has four sides. One of the sides can be employed for designating a crystal orientation. Designation of crystallographical orientations is a matter beyond the present invention. In the case of rectangular wafers which have four straight-line sides, the prevalent OF/IF marking which requires cut of two arcs on the periphery is inapplicable.
2. Description of Related Art
Processing of a GaN wafer comprises a grinding (lapping) step, a shaping (chamfering, bevelling) step, a gross polishing step, a fine polishing step, and a washing step. GaN is very rigid but fragile. Rigidity forces the processing to make use of silicon carbide (SiC) or diamond (C) polishing powder. A top surface of a GaN wafer is fine-polished into a mirror flat of surface roughness Ra less than 1 nm (Ra≦1 nm) by SiC or diamond fine powder. A bottom surface is lapped into a rugged texture of large roughness Ra like frosted glass by gross powder of a mesh from #400 to #1000. The obverse surface and the reverse surface have very different roughnesses. The roughness difference enables an operator to discern an obverse surface from a reverse surface by eyesight. At present obverse/reverse discrimination is done by the conspicuous difference of roughness observed by human eyesight in the case of rectangle GaN wafers. Since eyesight can discern tops from bottoms of rectangular wafers by the roughness, no obverse/reverse discriminating mark is formed on current rectangular GaN wafers.
In the case of round wafers, designation of crystal orientations requires an orientation flat (OF). On the contrary, in the case of rectangular wafers, one side can be assigned to a mark denoting crystal orientations by giving different lengths to two parallel pairs of four sides. Thus, current rectangle GaN wafers carry neither obverse/reverse marks nor orientation marks. This is a present state of technology of processing rectangle GaN wafers.
The most prevalent obverse/reverse discriminator is a set of an OF and IF for circular wafers of Si, GaAs or InP. There are some proposals of obverse/reverse marks other than the OF/IF discriminator. Some of the proposals are described.    {circle around (1)} Japanese Patent Laying Open NO.2-144908(JP0244908A), “Manufacture of semiconductor device”, proposed an obverse/reverse identification of a circular silicon (Si) wafer by chamfering top edges and bottom edges with different slanting angles and discerning a top from a bottom by the difference of chamfering angles. This relates not to a rectangle GaN wafer but to a round Si wafer. {circle around (1)} differs from the present invention in two preconditions.    {circle around (2)} Japanese Patent Laying Open NO. 60-167426(JP60167426A), “Semiconductor crystal wafer”, proposed another obverse/reverse and orientation discriminator of a circular silicon (Si) wafer by shooting a pulsed, focused laser beam on the silicon wafer along a line extending in a predetermined orientation, and forming a series of melted dots extending in the orientation. The direction shows crystal orientations and existence or non-existence of the melted dots denotes an obverse or reverse surface.    {circle around (3)} Japanese Patent Laying Open No.2000-331898(JP2000331898A), “Notched semiconductor wafer”, proposed a circular silicon wafer having an orientation mark by a small notch formed on a circular circumference and an obverse/reverse mark of differentiating bevelling angles formed in the notch. {circle around (3)} complained the OF/IF orientation caused asymmetric deviation of the center of gravity of a future 300 mm(12 inches) φ silicon wafer and invited accidental spin-off of the wafer from a spinning rotor.    {circle around (4)} Japanese Patent Laying Open No.58-71616(JP58071616A), “Manufacture of semiconductor device”, proposed an obverse/reverse discriminating mark of a circular silicon wafer with an asymmetrically bevelled circumference with different bevelling angles. {circle around (4)} is similar to {circle around (1)}. {circle around (4)} also relates not to a nitride rectangular wafer but to a circular silicon wafer.    {circle around (5)} Japanese Patent Laying Open No.8-316112(JP08316112A), “Semiconductor wafer with notch”, proposed an obverse/reverse mark of a large-sized circular silicon wafer. The obverse/reverse mark is a small notch formed on a circumference with different top and bottom bevelling angles. The position of the notch signifies crystal orientations. The difference of the bevelling angles discriminates between top and bottom. {circle around (5)} resembles {circle around (3)}.    {circle around (6)} Japanese Patent Laying Open No.2002-356398(JP2002356398A), “Gallium nitride wafer”, proposed a chamfered free standing gallium nitride wafer. {circle around (6)} declared that they had succeeded in making a small freestanding GaN plate. And {circle around (6)} said that they had bevelled an edge of the GaN plate by a rotary whetstone. {circle around (6)} related not to an obverse/reverse mark but to production of a GaN wafer.
A GaN single crystal is transparent for visible light. Available GaN single crystal substrates at present have a roughened reverse surface and a mirror-polished smooth obverse surface. The reverse surface looks like frosted glass. Human eye sight can discern the obverse surface from the reverse surface by the difference of the roughness. The roughness of the reverse surface causes several problems. Particles are apt to adhere to the roughened reverse surface. Some of the particles move around to the obverse surface in wafer processes and contaminate the obverse surface. The large difference of the roughness degrades flatness of the wafer and induces distortion. The wafer distortion causes errors of patterns in photolithography and causes low yield of products. Furthermore, the rugged reverse incurs occurrences of cracks in wafer processes. When the wafer is die-bonded upon a heat sink or a package, the rugged reverse surface reduces thermal conduction between the device and the heat sink/package and causes poor thermal diffusion.
Therefore, a request of mirror-flat, smooth reverse surfaces is made for GaN substrates. A flat reverse surface would reduce adhesion of particles on the reverse, would raise the heat-conduction for the heat sink and would lower the wafer distortion. If the reverse surface is polished into mirror flat, human eyesight cannot discern an obverse from a reverse. The difference of the roughness would act as an obverse/reverse discriminating mark no more. A new obverse/reverse discriminator should be required for nitride semiconductor wafers instead of the roughened reverses.
A first purpose of the present invention is to provide a rectangular nitride semiconductor single crystal wafer having an obverse/reverse discriminator without relying upon a roughened reverse surface. A second purpose of the present invention is to provide a rectangular nitride semiconductor single crystal wafer free from contamination of particles by eliminating reverse surface roughness which causes particle contamination. A third purpose of the present invention is to provide a rectangular nitride semiconductor single crystal wafer immune from a large convex or concave distortion by eliminating reverse surface roughness which causes wafer distortion. A fourth purpose of the present invention is to provide a rectangular nitride semiconductor single crystal wafer with high heat conductivity by eliminating reverse surface roughness which decreases heat conduction.
A rectangular nitride semiconductor wafer of the present invention has an obverse/reverse discriminator comprising a longer slanting edge and a shorter slanting edge formed at neighboring corners which align clockwise on an obverse surface.
Another rectangular semiconductor wafer has another obverse/reverse discriminator which is an asymmetric slanting edge which inclines to an obverse-counterclockwise neighboring side at a smaller slanting angle 5 degrees to 40 degrees and inclines to an obverse-clockwise neighboring side at a larger slanting angle 85 degrees to 50 degrees.
Another rectangular semiconductor wafer has another obverse/reverse discriminator which is different depths of chamfering.
Another rectangular semiconductor wafer has another obverse/reverse discriminator which is asymmetric characters written of an obverse surface or a reverse surface in normal or inverse posture.
An important contrivance of the present invention is obverse/reverse identification. Then, variances of obverse/reverse discriminating marks are described in detail by taking a rectangle GaN wafer as an example of nitride semiconductors.
A rectangle wafer has an obverse surface, a reverse surface and four sides. Two surfaces are anti-parallel. “Anti-parallel” means a kind of “parallel” in which normals are directed in reverse directions. This invention proposes a c-plane wafer. C-plane means a (0001) or (000-1) plane. The sides are all orthogonal to both obverse and reverse surfaces. Furthermore, sides are anti-parallel or orthogonal with each other, because the object is a rectangular wafer. When a (0001) wafer is an object, either (0001) or (000-1) plane is either an obverse surface or a reverse surface. When an obverse surface is determined to be the (0001) surface here, the reverse surface should be (000-1). When an obverse surface is determined to be (000-1), a reverse surface is (0001). This invention can be applied to both cases (0001) and (000-1) of an obverse surface. Thus, allusion to existence and orientation of a reverse surface is sometimes omitted in this description, because designation of an obverse surface can uniquely determine the orientation of a reverse surface.
The four sides are orthogonal to the obverse surface and the reverse surface. Planes or directions of nitride semiconductor crystals having hexagonal symmetry can be designated by a set of four Miller indices {hkmn}, (hkmn), [hkmn] or <hkmn>. Early three indices always satisfy a sum rule of h+k+m=0. The fourth index which denotes a c-axis part of an object plane is a unique one.
The wavy-bracketed {hkmn} is a collective plane representation. The round-bracketed (hkmn) is an individual plane representation. The key-bracketed <hkmn> is a collective direction representation. The square-bracketed [hkmn] is an individual direction representation. The individual direction [hkmn] is a normal standing on an individual plane (hkmn). Thus, the [hkmn] direction is vertical to the (hkmn) plane.
If two individual planes (h1 k1 m1 n1) and (h2 k2 m2 n2) or two individual directions [h1 k1 m1 n1] and [h2 k2 m2 n2] are orthogonal, an inner product is zero h1h2+k1 k2+m1 m2+n1 n2=0.
An object of the present invention is a rectangle wafer having two surfaces and four sides. Four sides are either anti-parallel or orthogonal. If one side is determined, other three sides are automatically determined. Since this invention restricts objects within c-plane wafers whose obverse is denoted by (0001) or (000-1), the four sides should have Miller indices (hkm0) of n=0. If one side is determined to be (h1 k1 m1 0), an opposite side is (−h1 −k1 −m1 0) due to antiparallelism and two neighboring sides are (h1 k1 m1 0) and (h2 k2 m2 0), where h1 h2+k1 k2+m1 m2=0. Hexagonal symmetry (h+k+m=0, n=0) allows four sides various Miller indices (11-20), (1-100), (22-40), (33-60), (2-200), (2-310), (2-530) and so on. If one side is (1-100), an opposite should be (−1100). An orthogonal side can be one of (11-20), (22-40), (33-60) and so on which are multiples of (11-20). Sets of (1-100) and (11-20), (1-100) and (22-40) . . . are allowable. Then, four sides which have two freedoms should be defined by determining two vertical planes. The present invention selects a set of (11-20) side and (1-100) side without choice of the multiples. Four sides are fully designated by determining two sides. In the description, orientations of two sides will be described instead of mentioning all four sides.
One of four sides is called a “reference side” which denotes crystallographical orientation exactly. Since this invention relates not to determining crystal orientations, the role of the reference side should be mentioned no more in detail.
Here, the reference side should be placed downside for identifying an obverse/reverse orientation. For the convenience of explanation, four sides are designated by a, b, c and d in a clockwise order on an obverse surface. Four corners are designated in series by α, β, γ and δ. The corners and sides align in a clockwise order of α, a, β, b, γ, c, δ and d on the obverse surface. They align in the same order counterclockwise on the reverse.
The present invention forms an obverse/reverse discriminator on a side opposite to the reference side. If the reference side is d-side, the opposite side which wears a mark is b-side. If c-side is a reference side, a-side is an opposite side.
Twelve types of the present invention are briefly described by examples.
[Type 1 (Clockwise Aligning Longer/Shorter Edges for (11-20) Reference (FIG. 1))]
A rectangular GaN wafer having a (0001) obverse surface, (11-20) and (1-100) sides, a longer slanting edge (L) and a shorter slanting edge (S) formed at neighboring corners, which align clockwise on the obverse surface, of a side opposite to a reference side (11-20). Lengths of the longer slant (L) and the shorter slant (S) should satisfy inequalities of K/40≦L≦K/12 and K/40≦S≦K/16, where K is a sum of lengths of four sides.
[Type 2 (Clockwise Aligning Longer/Shorter Edges for (1-100) Reference (FIG. 2))]
A rectangular GaN wafer having a (0001) obverse surface, (11-20) and (1-100) sides, a longer slanting edge (L) and a shorter slanting edge (S) formed at neighboring corners, which align clockwise on the obverse surface, of a side opposite to a reference side (1-100). Lengths of the longer slant (L) and the shorter slant (S) should satisfy inequalities of K/40≦L≦K/12 and K/40≦S≦K/16, where K is a sum of lengths of four sides.
[Type 3 (Asymmetric Edge for (11-20) Reference (FIG. 3))]
A rectangular GaN wafer having a (0001) obverse surface, (11-20) and (1-100) sides, an asymmetric slanting edge, which inclines to an obversely counterclockwise neighboring at Θ from 5 degrees to 40 degrees (5°≦Θ≦40°), at a left corner of a side opposite to a reference side (11-20).
[Type 4 (Asymmetric Edge for (1-100) Reference (FIG. 4))]
A rectangular GaN wafer having a (0001) obverse surface, (11-20) and (1-100) sides, an asymmetric slanting edge, which inclines to an obversely counterclockwise neighboring at Θ from 5 degrees to 40 degrees (5°≦Θ≦40°), at a left corner of a side opposite to a reference side (1-100).
[Type 5 (asymmetric obverse/reverse bevelling sides (FIG. 5))]
A rectangular GaN wafer having a (0001) surface, (11-20) and (1-100) sides which have a wider reverse bevelling width (h) and a narrower obverse bevelling width (g) (g<h). Favorable scopes of g and h are g=100 μm to 400 μm and h=300 μm to 1000 μm.
[Type 6 (asymmetric obverse/reverse round bevelling sides (FIG. 6))]
A rectangular GaN wafer having a (0001) obverse surface, (11-20) and (1-100) sides which have a wider reverse round bevelling width (h) and a narrower obverse round bevelling width(g) (g<h). Favorable scopes of g and h are g=100 μm to 400 μm and h=300 μm to 1000 μm.
[Type 7 (normal-postural characters on obverse surface (FIG. 7))]
A rectangle GaN wafer having a (0001) obverse surface, (11-20) and (1-100) sides and a series of characters written in normal posture in parallel with a reference side (11-20) in a [1-100] direction on the obverse surface by laser marking.
[Type 8 (normal-postural characters on reverse surface (FIG. 8))]
A rectangle GaN wafer having a (0001) obverse surface, (11-20) and (1-100) sides and a series of characters written in normal posture in parallel with a reference side (11-20) in a [1-100] direction on a reverse surface by laser marking.
[Type 9 (inverse-postural characters on reverse surface (FIG. 9))]
A rectangle GaN wafer having a (0001) obverse surface, (11-20) and (1-100) sides and a series of characters written in inverse posture in parallel with a reference side (11-20) in a [1-100] direction on a reverse surface by laser marking.
[Type 10 (normal-postural characters on obverse surface (FIG. 10))]
A rectangle GaN wafer having a (0001) obverse surface, (11-20) and (1-100) sides and a series of characters written in normal posture in parallel with a reference side (1-100) in a [11-20] direction on the obverse surface by laser marking.
[Type 11 (normal-postural characters on reverse surface (FIG. 11))]
A rectangle GaN wafer having a (0001) obverse surface, (11-20) and (1-100) sides and a series of characters written in normal posture in parallel with a reference side (1-100) in a [11-20] direction on a reverse surface by laser marking.
[Type 12 (inverse-postural characters on reverse surface (FIG. 12))]
A rectangle GaN wafer having a (0001) obverse surface, (11-20) and (1-100) sides and a series of characters written in inverse posture in parallel with a reference side (1-100) in a [11-20] direction on a reverse surface by laser marking.